Lessons from a disaster: Ten things we need to know about the Bohol quake

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Friday, October 25, 2013

TRAUMA and constant fear linger in most of the people who have been affected by the magnitude 7.2 earthquake that hit Central Visayas on October 15.

The deadly quake killed 211 as of 5 p.m. October 25, and the death toll is still slowly increasing. The damage it caused is already P1.45 billion worth of devastated buildings, bridges, and roads, among others. Thousands of families remain inside evacuation centers.

Ten days after the disaster and with more than 2,700 aftershocks recorded, has the public understood what happened, its implication and how to minimize the risks involved?

With the help of geoscientists from the Philippine Institute of Volcanology and Seismology (Phivolcs) and the University of the Philippines National Institute of Geological Sciences (UP NIGS), here are some of the things everyone needs to know about earthquake and what happened in Bohol.

Q: What happened in the October 15 earthquake in Bohol?

A: Around 8:12 a.m., residents in Central Visayas experienced ground shaking. The strongest, at Intensity VII, was felt in Tagbilaran City, Bohol and several areas in Cebu. Initially, Phivolcs said that the epicenter, or the location directly above the place that sends vibrations underneath the ground of the quake, was in Carmen town in Bohol. But after receiving other data, they relocated it to somewhere near Sagbayan.

Several aftershocks followed and are still occurring from time to time up to now.

INFOGRAPHICS: Bohol earthquake
[AS OF OCT. 21, 2013] Infographics by Rigil Kent Ynot of Sun.Star Cebu. Click on the image to ENLARGE.

Q: What caused the killer quake?

A: Earthquakes happen because rocks slip past each other. Dr. Mario Aurelio, a structural geologist from UP NIGS, said, "Ang nagko-cause ng movement ng blocks is that mayroong naiipong stress. When the stress is too strong for the fault to bear, bibigay na ang fault. In the process, 'yung paggalaw ng fault releases energy."

Aurelio said that the movement along a previously hidden fault caused the Bohol quake. The fault that Aurelio was discussing is the same fault that Phivolcs geologist Dr. Toto Bacolcol found a few days back.

"It's better to call it an undiscovered fault. Parang something that has already existed but nobody discovered it and though it has been discovered, ang impression ng tao is bago, pero sa totoo lang, walang bago... Thanks or no thanks to the earthquake, alam na natin na mayroon palang fault sa ilalim ng Bohol," said Aurelio.

Dr. Artemio Daag of the Phivolcs Geology division said, "Mayroon na sila ngayong mga around one kilometer then sinusundan pa nila iyan. Nilalakad talaga to locate."

Daag also clarified that not all cracks in the land are faults and at the same time, not all faults can be seen by the naked eye.

"We map based on morphology. So kung walang morphology or nabura through erosion, hindi mo makikita," said Daag.

He added, "Hindi naman necessarily na buong stretch ay lalabas."

Phivolcs already added several seismographs on different areas in Bohol to have a better understanding of the fault that caused the earthquake.

Daag said, "Nagrequest kami ng dagdag na satellite images... Nagrequest din kami ng aerial survey para sundan ang fault. Pero mahirap din talaga sundan kasi lalo 'yung mga researchers on the ground, ang sabi nila bulubundukin ang area."

Q: Why is the damage so massive?

A: According to the United States Geological Survey (USGS), a magnitude 7.2 quake can generate as much as an Intensity VIII tremor, which means: "Damage slight in specially designed structures; considerable damage in ordinary substantial buildings with partial collapse. Damage great in poorly built structures. Fall of chimneys, factory stacks, columns, monuments, walls. Heavy furniture overturned."

Q: Is Bohol sinking?

A: The fault that generated the temblor is what geologists call a "thrust fault." (See USGS animation here.) Instead of moving horizontally, the blocks move upward and downward relative to each other.

Following this, Bohol is not sinking.

As a result, Bohol gained hundreds of meters of shoreline. "Umatras 'yung dagat. Hindi naman magsusubside 'yung dagat. Same volume iyan, hindi nagbabago," said Aurelio. He clarified that the land uplifted, causing the sea to recede by more than 100 meters.

Sinkholes in Catigbian, Bohol after quake
YELLOW RIBBON. A police officer cordons off a road, yard and house crossed by a large gash that appeared on the ground in Bagtic, Catigbian, Bohol during the 7.2-magnitude earthquake last October 15. Several large sinkholes have also appeared in the town, at least one of them revealing blue-green water below. (Sun.Star Cebu photo/Alex Badayos)

Q: How about sinkholes? Don't they mean that Bohol is sinking due to the quake?

A: Daag said that Bohol island is largely made up of limestones while a few portions of remnant of old volcanoes. Aurelio estimated Bohol to be roughly 60 percent made up of limestones, including the Chocolate Hills.

Imagine making a sand castle and you were not able to compact the sand before forming it into a castle. Its foundation will be weak and even just a faint movement will cause it to collapse.

Daag said, "Ibig sabihin, dating nakalubong sa dagat ito, tapos umangat lang. Tapos kapag naexpose na, 'yung mga rainwater ang magfoform ng... carbonic acid so nagdi-dissolve 'yung ilalim minsan. Parang 'yung mga kweba. Marami ring mga kweba diyan. So kung mayroon ganoong structural indicators, pag niyanig mo iyan, pwedeng mag-collapse iyan."

Aurelio added, "Bohol is very prone to sinkholes because limestones are also prone to cave formation. They react to water over long periods of time. Ngayon, pag may cavity ka na sa ilalim, ibig sabihin siyempre, weakness na iyan. Anything above it lalo na kapag medyo manipis-nipis na, especially during an earthquake, then you induce a sinkhole formation."

The two scientists emphasized that most reported sinkholes in Bohol are localized and that they can occur even without the earthquakes. [Read: New sinkholes scare villages]

Q: Has Bohol ever been prone to earthquakes of this magnitude?

A: Aurelio said, "A record (seismological records) shows that Bohol has not experienced as many earthquakes as say, Luzon like Ilocos which is frequently visited by earthquakes. Ang Bohol, relatively, hindi siya expected na maraming earthquakes."

But Phivolcs said in its website, "Bohol Island is one of the seismically active areas in the country. Instrumental monitoring of earthquakes for the past century has detected many small to moderate-magnitude earthquakes in Bohol Island. There is at least one known earthquake generator on the island, the East Bohol Fault. In addition, there are other local faults that can be sources of small to large magnitude earthquakes."

In 1990, Phivolcs recorded a magnitude 6.8 earthquake caused by another reverse fault offshore. Sixteen towns felt Intensity VIII shaking, leaving severe damage to properties, numerous deaths, and localized tsunami inundation.

Q: Will a strong quake happen again in Bohol anytime soon?

A: While Phivolcs said that the province and some nearby areas are still expected to experience aftershocks as the surface of the Earth is still adjusting to the massive movement of the October 15 quake, in a lifetime, it is highly unlikely that a fault will generate an earthquake of the same magnitude.

Aurelio said, "So kung nag-release siya ng stress, ibig sabihin this place will not experience earthquake that magnitude again at least within the period na kung tawagin ay recurrence interval."

Recurrence interval is the time measured between two huge earthquakes caused by a single fault.

Recurrence interval, according to Aurelio and Daag, can be within several hundreds, thousands or even millions of years.

"Kapag nag-earthquake ang isang lugar of that magnitude, hindi na iyan mag-eearthquake at least within the lifetime of that person," said Aurelio.

Q: Why can't we not predict earthquakes?

A: Earthquakes caused by movements of the Earth, as opposed to volcanic eruptions, cannot be "predicted" because of the very nature of the Earth. The Earth's crust is like a jigsaw puzzle where the pieces constantly move.

Scientists all over the globe have been trying to find ways to predict when the next earthquake would occur but the best they can do as of now is to tell the likelihood that an earthquake will occur based on previous data. Daag called this as "medium-term assessment."

"Alam mo lang na active ang region pero in 10 years, five years, mahirap sabihin," said Daag.

But geoscientists and seismologists are able to identify which areas are seismically active or are experiencing more earthquakes than other areas.

Q: How do we minimize the damage and risks involved in earthquakes?

A: First, the public should be informed about what to do during earthquakes. Phivolcs also conducts information campaigns and assistance during earthquake drills.

At the same time, existing structures should be strengthened. There is a process called retrofitting where engineers reinforce structures to stand greater pressure.

But in case you are planning to build a house or a building, you can ask Phivolcs to assess whether it is near an active fault. Follow the National Building Code. You cannot build anything within five meters of a fault.

Q: So what should people be worried about?

A: The October 15 Bohol quake was caused by a previously hidden fault. The magnitude 6.9 earthquake that shook Negros Oriental in 2012 happened in an area where no active faults were known to exist.

Aurelio said, "The question is have all the faults been mapped? Of course, the answer is no."

"So we fear the unknown... If we see something, alam natin ang gagawin. As far as faults are concerned, we should pay more attention to those that we do not see," added Aurelio. (Sunnex)

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